Conventional test patterns comprise a cylindrical object that is transparent to x-rays, which object is equipped on its surface with radio-opaque balls forming the markers. The balls are conventionally spaced out along a spiral. However, this type of arrangement does not allow many balls to be integrated into the cylindrical object, this adversely affecting the precision of the calibration and therefore the reconstruction of the 3D image.
The test pattern is conventionally equipped with a reference ball having a different size from the other balls. The positions of the other balls relative to this reference ball are known. It is the identification and the position of the projection of this reference ball in a 2D image, knowledge of the positions of the other balls with respect to the reference ball and the positions of the projections of the other balls with respect to the projection of the reference ball that allow the positions of the projections of all the balls in the image and their positions in the 3D space to be matched and thus the projection matrix to be obtained by means of suitable algorithms. The presence and identification of the projection of the reference ball in the projection of the object is therefore of fundamental importance. However, if the projection of this ball is superposed on that of another ball in an image, this possibly being the case for certain viewing angles, if the projection of this ball does not appear in the image, this possibly being the case if the image of the test pattern is truncated, or indeed if the image is of poor quality, it is then possible to attribute, to a projection detected in a 2D image, the wrong ball of the 3D space, or to find it impossible to attribute the corresponding balls to the projections. The calibration is then impossible or erroneous. In other words, this type of test pattern leads to calibrating methods that lack robustness with respect to erroneous identification of the projection of the reference ball (false detections). Moreover, it may prove to be complex and expensive to identify, with a good confidence level, the projection of the reference ball from a set of projections of balls.
A test pattern that does not require a reference ball to be identified is known from United States patent U.S. Pat. Ser. No. 6,715,918. This test pattern comprises a cylindrical body and markers arranged along a spiral. The markers comprise two types of markers that are differentiated by their geometric properties, for example their respective sizes and/or shapes. A binary value is associated with each of the two types of physical markers. The values assigned to a sequence of successive markers then form a binary code. Markers of the first and second type are arranged in a spiral so that when a preset number of successive markers is considered, the binary code obtained appears only once along the spiral whatever the read-out direction. However, this test pattern has a certain number of drawbacks. Specifically, since the markers are differentiated by their geometric properties, and especially by their sizes, the closer a marker is to the x-ray source, the larger its projection. Therefore, it is not possible to guarantee, even if markers of very different sizes are provided, that the projections of the markers of the two types will be easily differentiable at each and every angle of image capture. There is therefore a nonzero risk that they will be mixed up in the projections obtained and an image of a marker of one type attributed to a marker of another type. This limits the reliability of the calibration.
The reliability of the calibration is also limited because of the spiral arrangement of the markers. When a light source illuminates the test pattern along a central axis perpendicular to the axis of the cylinder, the distances between the projections of the markers illuminated by the edges of the beam emitted by the source are very small and the images of these markers may be superposed. This makes the attribution of a projection of a marker to a given marker of the test pattern more difficult and less reliable, because of the complexity of the spiral shape, and limits the precision of the calibration. One solution given in patent U.S. Pat. Ser. No. 6,715,918 is to exclude the edges of the cylinder from the projection zone so as to prevent superposition of the projections of the markers. However, this solution, because it excludes certain markers from the projections, leads to a limited precision. Another solution given in patent U.S. Pat. Ser. No. 6,715,918 is to make provision for a marker-detecting algorithm allowing the projections of markers located on the edges of the cylinder with respect to the central axis to be excluded from the calibration. However, this increases the complexity of the calibration and is not 100% reliable.